I think there are a lot of options that could work for you depending on your needs.

If you have no idea what the population is, can't estimate how many participants or where they are coming from:
I would probably give all participants a anonymous link to sign up for the study and to fill out the consent form. Use the Contact Trigger function to create a panelist. Then send surveys to that panel but anonymize the survey project. This way you only collect their information on the initial sign up, can prove they opted in by pulling the first project or panelist data, but it never gets tied to subsequent data collection.
You would want to "redirect" at the end of a project for which a prize is awarded to a second project to collect PII once again to know who "finished" for the prize raffle.